In the past procedures have been proposed for the conversion of fibers formed from acrylic polymers to a modified form possessing enhanced thermal stability. Such modification has generally been accomplished by heating a fibrous material in an oxygen-containing atmosphere at a moderate temperature for an extended period of time.
U.S. Pat. Nos. 2,913,802 to Barnett and 3,285,696 to Tsunoda disclose processes for the conversion of fibers of acrylonitrile homopolymers or copolymers to a heat resistant form. The stabilization of fibers of acrylonitrile homopolymers and copolymers in an oxygen-containing atmosphere involves (1) a chain scission and oxidative crosslinking reaction of adjoining molecules as well as (2) a cyclization reaction of pendant nitrile groups. It is generally recognized that the rate at which the stabilization reaction takes place increases with the temperature of the oxygen-containing atmosphere. However, the stabilization reaction must by necessity be conducted at relatively low temperatures (i.e. below about 300.degree. C.), since the cyclization reaction is exothermic in nature and must be controlled if the original fibrous configuration of the material undergoing stabilization is to be preserved. Accordingly the stabilization reaction tends to be time consuming, and economically demanding because of low productivity necessitated by the excessive time requirements. Prior processes proposed to shorten the period required by the stabilization reaction include that disclosed in U.S. Pat. No. 3,416,874. Additionally, see the processes of commonly assigned United States Ser. Nos. 777,901 (now U.S. Pat. No. 3,592,595)and 777,902 (now U.S. Pat. No. 3,647,770), filed Nov. 21, 1968 of Klaus H. Gump and Dagobert E. Stuetz. See also, our application Ser. No. 109672, Jan. 25, 1971 (now U.S. Pat. No. 3,708,326) filed, entitled "Stabilization of Acrylic Fibers and Films" wherein a previously formed acrylic fibrous material or film is impregnated with a stabilization promoting agent by contact with a solution of the same provided at a moderate temperature, dried to remove the solvent, and heated in an oxygen-containing atmosphere at a more highly elevated temperature until a stabilized fibrous material or film is formed.
While stabilized acrylic fibrous materials may be used directly in applications where a non-burning fiber is required, demands for the same have been increasingly presented by manufacturers of carbonized fibrous materials. Carbonized fibrous materials are commonly formed by heating a stabilized acrylic fibrous material in an inert atmosphere, such as nitrogen or argon, at a more highly elevated temperature. During the carbonization reaction elements such as nitrogen, oxygen, and hydrogen are substantially expelled. Accordingly, the term "carbonized" as used in the art commonly designates a material consisting of at least about 90 percent carbon by weight, and generally at least about 95 percent carbon by weight. Depending upon the conditions under which a carbonized fibrous material is processed, it may or may not contain graphitic carbon as determined by the characteristic x-ray diffraction pattern of graphite. See, for instance, commonly assigned United States Ser. No. 777,275, filed Nov. 20, 1968 (now abandoned) of Charles M. Clarke for a preferred procedure for forming carbonized and graphitized fibrous materials from a stabilized acrylic fibrous material.
It is an object of the invention to provide an improved process for forming thermally stabilized shaped acrylic articles.
It is an object of the invention to provide an improved process for forming flame-proofed fibrous materials or films derived from acrylic polymers.
It is an object of the invention to provide a process wherein the thermal stabilization of an acrylic fibrous material or film is accelerated.
It is another object of the invention to provide an improved process for forming stabilized fibrous materials or films derived from acrylic polymers which results in a product which is suitable for carbonization, or carbonization and graphitization.
These and other objects, as well as the scope, nature, and utilization of the invention will be apparent from the following detailed description and appended claims.